THE CLAIM: Chronically bad skin or the occasional nasty breakout can be a major source of stress. That much is known. But what about the reverse? Although scientists have suspected for some time that emotional stress can cause or worsen acne, the evidence for the claim has mostly been weak, stemming largely from anecdotal reports and a few small studies.

THE FACTS: But now the answer appears fairly clear. A new study, the largest on the subject to date, was able not only to confirm the relationship, but also to provide a possible explanation.

The study, conducted by researchers at Wake Forest University, followed 94 high school students with mild or moderate acne for several months. Acne is generally associated with high levels of sebum, the oily substance that coats the skin and protects the hair. Because sebum levels are known to wax and wane with variations in weather, the study was conducted where the temperature and humidity rarely change, Singapore.

Using a standard measure of stress, the researchers showed that in periods of high emotional strain, like before major exams, the students were 23 percent more likely to experience breakouts. At the same time, their sebum production did not vary much whether they were experiencing high or low stress, indicating that levels of the substance had little or no role. Instead, the results may have more to do with inflammation. Other studies have shown that stress can provoke inflammation, and acne is an inflammatory disease.

THE BOTTOM LINE: Research suggests that stress can in fact lead to bad skin.